Flying leads for integrated circuits

ABSTRACT

A method and apparatus for interconnecting electronic circuits using nearly pure soft annealed gold mechanically compressed within through-plated holes. The invention has its application in attaching integrated circuit dice directly to circuit boards by ball bonding gold wires to the bonding pads of the integrated circuit dice in a substantially perpendicular relationship to the surfaces of the dice and inserting the gold leads into through-plated holes of circuit boards which provide an electrical and a mechanical connection once the leads are compressed within the through-plated holes. The present invention also finds its application in the interconnection of sandwiched circuit board assemblies where soft gold lead wires are inserted into axially aligned through-plated holes of the circuit boards and compressed so that the gold lead wires compress and buckle within the through-plated holes, forming an electrical connection between the circuit boards.

This Application is a continuation of copending patent application Ser.No. 07/376,156 filed Jun. 30, 1989, now abandoned. Patent applicationSer. No. 07/376,156 was a continuation of application Ser. No.07/243,591 filed Sep. 12, 1988, now abandoned. Application Ser. No.07/243,591 was a division of application Ser. No. 07/053,142, filed May21, 1987, now U.S. Pat. No. 5,054,192.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of electrical circuitinterconnect, and more specifically to a new apparatus and method forhigh-density packing and interconnect of integrated circuits on printedcircuit (PC) boards and PC boards on PC boards.

BACKGROUND OF THE INVENTION

Integrated circuits are typically fabricated on wafers which are cutinto individual integrated circuits and packaged within hermeticallysealed ceramic or plastic packages. The signal and power lines from theintegrated circuit are brought out to the pins of the package by meansof leads attached to the bonding pads on the integrated circuit chips.The chips are then used to form larger circuits by interconnecting theintegrated circuit packages by means of PC boards. The circuit boardscontain interconnect lines or foils on the surfaces of the circuitboards or within planar layers The circuit board is populated withintegrated circuit packages which are soldered to plated via holes or onsurface mounted pads on the circuit board. The soldering process formsan electrical and mechanical connection between the integrated circuitpackage and the circuit board.

To form larger circuits, circuit boards populated with integratedcircuit packages are interconnected by a variety of connectors, jumperwires, or cables. The physical arrangement of the circuit boards inrelation to one another is also accomplished in a wide variety ofconfigurations. One popular highdensity interconnect technique is tostack the circuit boards in a sandwiched relationship to one another andelectrically interconnect the circuit boards with interboard connectors.This packing technique achieves a fair amount of packing density,limited by the interboard spacing requirements of heat dissipation andconnector spacing.

The aforementioned technique of forming larger circuits from individualintegrated circuits using integrated circuit packages and circuit boardsresults in limited packing density of the actual area which is used forelectrical circuits. The actual integrated circuit chips themselves aretypically smaller than onetenth of a square inch, and in total wouldcover only 10-20 percent of the board area. However, due to theinefficiencies of packaging of integrated circuit chips and connectingthe integrated circuit chips to the circuit boards, it is difficult orimpossible to increase the packing density on circuit boards to improvespeed or spacing advantages. In addition, inter-board spacing is limitedby the area consumed by the integrated circuit packages and inter-boardconnects. This limited packing density limits the inter-circuit signalspeed due to the long propagation delays along the long interconnectlines.

The present invention provides a new apparatus and method forhigh-density interconnect of integrated circuit chips on circuit boardsand between circuit boards which overcomes the wasted space and speeddisadvantages of the prior art.

SUMMARY OF THE INVENTION

The present invention provides for placing unpackaged integrated circuitchips directly on circuit boards by using soft gold lead wires attachedto the bonding pads to form the mechanical and electrical connectionbetween the integrated circuit chips and the circuit boards. The presentinvention also provides for interconnection of sandwiched circuit boardsby using soft gold jumper wires connected through the throughplatedholes of the circuit boards.

Gold wires comprising nearly pure soft annealed gold are ball-bonded tothe bonding pads of integrated circuit chips, and the soft gold wiresare stretched to a substantially perpendicular position with respect tothe surface of the integrated circuit chip, forming flying leads. Thecircuit boards to which the integrated circuit chips are to be attachedare manufactured with plated holes having hole patterns substantiallymatching the bonding pad patterns of the integrated circuit chips. Theintegrated circuit chips with the flying leads are then positionedfacing the circuit board and the flying leads are inserted through theplated holes such that the flying leads partially protrude from thecircuit board. Caul plates are then positioned on the outer sides ofthis sandwich and pressed together so that the sticky or soft gold ofthe flying leads is compressed within the plated holes, causing the softgold to deform against the surface of the plated holes and therebyforming a strong electrical and mechanical bond. The caul plates arethen removed and the integrated circuit package remains firmly attachedto the circuit board. This results in improved packing density ofintegrated circuit chips on circuit boards.

Gold wires comprising nearly pure soft annealed gold are insertedthrough axially aligned plated holes of two or more circuit boards in asubstantially perpendicular direction to the planar surface of theboards. The gold wire is selected to be slightly longer than thedistance through the axially aligned holes such that a portion of thewires protrude through one or both sides of the sandwich of circuitboards. Caul plates are then placed on the outer sides of the circuitboard sandwich and pressed together so that the soft gold is compressedwithin the plated holes, causing the soft gold to deform against theinner surface of the plated holes to form an electrical connectionbetween the circuit boards.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, where like numerals identify like components throughoutthe several views,

FIG. 1 a wide view of an integrated circuit die onto which flying goldleads are ball bonded and straightened by a ball bonding machine.

FIG. 2 shows the six steps that the flying lead ball bonder performs inorder to attach a flying lead to an integrated circuit die.

FIG. 3 shows the bonding pad pattern on a typical integrated circuitalong with the corresponding plated hole pattern on a circuit boardwhich mates the integrated circuit chip to the circuit board.

FIG. 4 shows the relative positions of the integrated circuit chip andthe circuit board prior to compression of the flying leads into theplated holes.

FIG. 4a is a closeup view of a single ball-bonded flying lead prior tocompression within the plated hole of the board.

FIG. 5 shows the relative positions of the integrated circuit chip andthe circuit board after the flying leads have been compressed inside theplated holes of the circuit board.

FIG. 5a is a closeup view of a ball-bonded flying lead that has beencompressed into a plated hole on the circuit board.

FIG. 6 is a larger view of the compression process whereby a pluralityof integrated circuit chips having flying leads are attached to a singleprinted circuit board through the application of seating force on caulplates which sandwich the circuit board/chip combination.

FIG. 7 is a plated hole pattern for a typical PC board onto whichintegrated circuit dice are attached in the preferred embodiment of thepresent invention.

FIG. 8 is a module assembly onto which a plurality of circuit boardspopulated with integrated circuit chips are placed.

FIG. 9 is a side view of the module assembly of FIG. 8 showing thedetails of the logic jumpers and power jumpers for logic and powerinterconnection between the sandwich assembly of printed circuit boards.

FIG. 10 is a closeup view of a single logic jumper that has beencompressed within the axially aligned plated holes of the sandwichedprinted circuit boards of the module assembly of FIG. 9.

FIG. 11 shows the first two steps of the pressing operation forcompressing the logic jumpers in a single stack assembly of PC boards ona module assembly.

FIG. 12 shows the second two steps of the compression of the logicjumpers on a single stack assembly of four printed circuit boards on amodule assembly.

FIG. 13 shows the method of compressing the power jumpers through theassembly of stacked printed circuit boards.

FIG. 14 shows the compression of the gold posts between the power platesand the power blades of the module assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment of the present invention relates to thehigh-density packing of silicon or gallium arsenide (GaAs) integratedcircuit chips on single-layer or multi-layer interconnect printedcircuit boards with plated-through via holes and the high-densitypacking of circuit boards in a sandwiched arrangement. The applicationof this technology is designed for speed improvements, improved heatdissipation, and improved packing density required for modernsupercomputers such as the Cray-3 manufactured by the assignee of thepresent invention.

The placing of the integrated circuit chips or dice directly on thecircuit boards eliminates the bulky packaging normally found onintegrated circuits and typically referred to as DIPs (dual inlinepackages), SIPs (single inline packages), SMDs (surface mount devices),leadless chip carriers, and the like. All of the aforementioned packagesconsume valuable circuit board real estate and in turn cause increasedpropagation delay between active circuits due to long signal paths. Inaddition, the aforementioned circuits present heat dissipation barrierswhich vary with the thermal conductivity of the packaging material. Byremoving the chips from the packages and placing them directly on thecircuit boards, the integrated circuit chips or dice can be surroundedby liquid coolant to improve cooling.

Flying Lead Construction

FIG. 1 shows the preferred embodiment for attaching the flying goldleads to the silicon or gallium arsenide packaged chip or die beforeattaching the die to the circuit board. The leads are made of soft goldwire which is approximately 3 mils in diameter. The GaAs chips used inthe preferred embodiment contain 52 bonding pads which have a sputteredsoft gold finish. The objective of the die bonding operation is to forma gold-to-gold bond between the wire and the pad. A Hughes automaticthermosonic (gold wire) ball bonding machine Model 2460-II is modifiedin the preferred embodiment to perform this operation, and is availablefrom Hughes Tool Company, Los Angeles, Calif. This machine was designedand normally used to make pad-to-lead frame connections in IC packagesand has been modified to perform the steps of flying lead bonding asdescribed below. The modifications include hardware and software changesto allow feeding, flaming off, bonding and breaking heavy gauge goldbonding wire (up to 0.0030 dia. Au wire).

The Hughes automatic ball bonding machine has an X-Y positioning bedwhich is used to position the die for bonding. The die is loaded on thebed in a heated vacuum fixture which holds up to 16 dice. The Hughesbonding machine is equipped with a vision system which can recognize thedie patterns without human intervention and position each bonding padfor processing. An angular correction as well as an X-Y position isavailable to the machine.

The soft gold wire that is used for the flying leads in the preferredembodiment of the present invention is sometimes referred to as stickygold or tacky gold. This gold bonding wire is formed from a 99.99%high-purity annealed gold. The process of annealing the high-purity goldresults in a high elongation (20-25% stabilized and annealed), lowtensile strength (3.0 mil., 50 gr. min.) gold wire which is dead soft.The wire composition (99.99% pure Au non-Beryllium doped) is as follows:

    ______________________________________                                        Gold                  99.990% min.                                            Beryllium              0.002% max.                                            Copper                 0.004% max.                                            Other Impurities (each)                                                                              0.003% max.                                            Total All Impurities   0.010% max.                                            ______________________________________                                    

This type of gold is available from Hydrostatics (HTX grade) orequivalent.

Referring to FIG. 1, the flying lead die bonding procedure begins withthe forming of a soft gold ball at the tip of the gold wire. The wire isfed from a supply spool (not shown) through a nitrogen-filled tube 109(shown in FIG. 2) to a ceramic capillary 100. The inside of thecapillary is just slightly larger than the wire diameter. The nitrogenin the connecting tube 109 can be driven either toward the capillary oraway from the capillary toward the supply spool. This allows the goldwire to be fed or withdrawn from the capillary tip.

The gold ball 106 formed at the end of the gold wire 101 isthermosonically bonded to bonding pad 105 of chip 104. The capillary tip102 of capillary 100 is capable of heating the ball bond to 300° C.concurrent with pressing the ball 106 onto the pad 105 and sonicallyvibrating the connection until a strong electrical and mechanicalconnection is formed. The capillary 100 is then withdrawn from thesurface of chip 104 and the wire 101 is extruded from the tip 102. Anotching the specific notching operation described herein, is to make anotch 107 at the appropriate height of the flying lead to break theconnection and to stiffen the lead. Wire clamp 108 grasps the gold wire101 and the capillary is withdrawn upward, breaking the flying lead at107 and concurrently performing a nondestructive test of the ball bondto bonding pad connection.

The sequence of steps required to make a flying lead bond to the packagedie is shown in FIG. 2. Step 1 begins with the feeding of apredetermined amount of wire through the capillary 100. A mechanical armthen positions an electrode 114 below the capillary tip 102 and ahigh-voltage electrical current forms an arc which melts the wire andforms a gold ball with a diameter of approximately 6 mils. This istermed electrostatic flame-off (EFO). Specified ball size range isattainable through EFO power supply output adjustment up to 10milliamps. During this step, the clamps 108 are closed and the nitrogendrag is off. This action occurs above the surface of the integratedcircuit chip so as to avoid any damage to the chip during the EFO ballforming process.

In step 2, the nitrogen drag 109 withdraws the supply wire 101 into thecapillary 100 and tightens the ball against the capillary tip 102.

The capillary tip 102 is heated to 200° C. (range of ambient to 300° C.)to assist in keeping the gold wire 101 in a malleable state. The diefixture is also heated to 200° C. (range of ambient to 300° C.) to avoidwire cooling during the bonding process. The die fixture is made ofTeflon-coated aluminum. As shown in FIG. 1, a vacuum cavity or vacuumplate 103 holds the chip 104 in position on the fixture during thebonding process.

In step 3, the bonding machine lowers the capillary 100 to the surfaceof a bonding pad 105 and applies high pressure (range of 30-250 grams)to the trapped gold ball 106 along with ultrasonic vibration at thecapillary tip 102. The capillary tip 102 is flat, with a 4-mil insidediameter and an 8-mil outside diameter. The ball 106 is flattened toabout a 3-mil height and a 6-mil diameter. Ultrasound is driven throughthe ceramic capillary 100 to vibrate the gold ball 106 and scrub thebonding pad surface. The sound is oriented so that the gold ball 106moves parallel to the die surface. The Hughes ball bonding machine hasthe ability to vary the touch-down velocity, i.e., soft touch-down forbonding GaAs, which is program selectable. The ultrasonic application isalso program selectable.

In step 4, the capillary 100 is withdrawn from the surface of chips 104,extending the gold wire 101 as the head is raised. The nitrogen drag isleft off and the capillary is raised to a height to allow enough goldwire to form the flying lead, a tail length for the next flying lead,and a small amount of clearance between the tail length and thecapillary tip 102. The Hughes ball bonder device is capable of selectingthe height that the capillary tip can move up to a height ofapproximately 0.750 inch.

In step 5, an automatic notching mechanism 115 moves into the area ofthe extended gold wire 101 and strikes both sides of the wire with steelblades. This is essentially a scissor action which cuts most of the waythrough the gold wire 101, forming a notch 107. The notch 107 is made 27mils above the surface of the die. The notching mechanism has been addedto the Hughes ball bonder for the precise termination of the flyingleads. The Hughes ball bonder has been modified to measure and displaythe notch mechanism height. The activation signal for the notchmechanism is provided by the Hughes ball bonder system for the properactivation during the sequence of ball bonding. The flying lead lengthis adjustable from between 0.0 mils to 50.0 mils. It will be appreciatedby those skilled in the art that the notching function can beaccomplished with a variety of mechanisms such as the scissor mechanismdisclosed above, a hammer-anvil system, and a variety of othermechanisms that merely notch or completely sever the wire 101.

In step 6, clamp 108 closes on the gold wire 101 above the capillary 100and the head is withdrawn until the gold wire breaks at the notchedpoint. This stretching process serves several useful purposes Primarily,the gold wire is straightened by the stretching force and standsperpendicular to the die surface. In addition, the bond isnon-destructively pull-tested for adhesion at the bonding pad. The lead101 is terminated at a 27-mil height above the surface of chip 104 inthe preferred embodiment. At the end of step 6, the capillary head forthe bonding mechanism is positioned over a new bonding pad and theprocess of steps 1-6 begin again. The bonding wire 101 is partiallyretracted into the capillary once again, and the clamps are closed, asshown in step 1, so that a new ball may be formed by the EFO.

The die positions are roughly determined by the loading positions in thevacuum fixture. The Hughes automatic bonding machine is able to adjustthe X-Y table for proper bonding position of the individual die. Anangular correction is automatically made to adjust for tolerance inplacing the die in the vacuum fixture. This is done through a visionsystem which recognizes the die pad configurations. Using the modifiedHughes automatic bonding machine with the current bonding technique, aminimum bonding rate of 2 die pads per second is possible.

Circuit Board Construction

Once the gold bonding leads are attached to the integrated circuit chipor die, the chip is ready to be attached to the circuit board. As shownin FIG. 3, the bonding pattern of the integrated circuit chip 104matches the plated hole pattern on the circuit board 110. For example.,the top view of integrated circuit chip 104 in FIG. 3 shows the bondingpad 105 in the upper right corner. The circuit board 110 shown in FIG. 3shows a corresponding plated hole 111 which is aligned to receive thebonding lead from bonding pad 105 when circuit board 110 is placed overintegrated circuit chip 104 and the flying leads are inserted into thehole pattern on the circuit board. Thus, each bonding pad of integratedcircuit chip 104 has a corresponding plated hole on circuit board 110aligned to receive the flying leads.

The circuit board assembly operation begins with the die insertion inthe circuit board. The circuit board is held in a vacuum fixture duringthe insertion process. This is to make sure that the board remains flat.Insertion can be done by hand under a binocular microscope or productionassembly can be done with a pick-and-place machine.

Referring to FIG. 4, the circuit board 110 with the loosely placed chip104 is mounted on an aluminum vacuum caul plate (lower caul plate) 113.Steel guide pins (not shown) are placed in corner holes of the circuitboard to prevent board motion during the assembly operation. A second(upper) caul plate 112 is then placed on the top side of the circuitboard populated with chips to press against the tops (non-pad side) ofthe chips 104. The sandwich assembly comprising the circuit circuitboard, the chip and the caul plates is then placed in a press andpressure is applied to buckle and expand the gold leads 101 in theplated holes 111 of the circuit board.

The side view of the sandwiched circuit board 110, integrated circuitchip 104, and caul plates 112 and 113 in FIGS. 4 and 5 illustrates theposition of the gold leads 101 before and after the pressing operation,respectively. In the preferred embodiment there is a 7-mil exposure ofgold lead 101 which upon compression will buckle and expand into theplated hole 111 of the circuit board 110. The 3-mil diameter wire 101 ina 5-mil diameter hole 111 means the initial fill is 36 percent of theavailable volume. After pressing, the fill has increased to 51 percentas a result of the 7-mil shortening of the gold lead 101. As shown ingreater detail in FIGS. 4a and 4b, The lead typically buckles in two ormore places, and these corners are driven into the sides of the platedhole 111 of the circuit board. The assembly is completed in one pressingoperation. The circuit board 110 can now be removed from the press withthe integrated circuit chip 104 securely attached and electricallybonded to the plated holes of the circuit board.

FIG. 6 shows a broader view of the circuit board press which is used toattach the integrated circuits to the printed circuit board. The uppercaul plate 112 is a Teflon-coated seating caul plate which is alignedthrough alignment pins 114 with the circuit board 110 and the lower caulplate 113, which is a vacuum caul plate, to hold the circuit board flatduring the pressing process. The alignment pins 114 are used to preventthe printed circuit board 110 from sliding or otherwise moving duringthe pressing process. A seating force is applied to the top of uppercaul plate 112 which forces the excess flying lead material into theplated holes of printed circuit board 110. Thus, integrated circuitchips 104 are mechanically and electrically bonded to printed circuitboard 110.

It will be appreciated by those skilled in the art that many variationsof the above-described pressing operation can be used which result inthe same or equivalent connection of the flying leads to the PC boards.For example, the flying leads of the chips could be completely insertedinto the through-plated holes of the PC board prior to the pressingoperation with the excess gold leads protruding out the opposite side.The first caul plate could then be used to hold the chip onto the PCboard while the second caul plate is used to compress the leads into theholes.

Module Assembly Construction

A sandwiched assembly of printed circuit boards populated withintegrated circuit chips is interconnected using a technique similar tothat used in bonding the integrated circuit chips to the circuit boards.As is more fully described below, soft gold wires are inserted throughaxially aligned plated holes between layered circuit boards which arecompressed using caul plates to partially fill the plated holes with thesoft gold wires to form an electrical connection substantiallyperpendicular to the planar surfaces of the printed circuit boards.

FIG. 7 is an example of a printed circuit board hole pattern for thetype of circuit boards used in the Cray-3 computer manufactured by theassignee of the present invention. In the preferred embodiment of thepresent invention, each circuit board provides 16 plated hole patternsfor the acceptance of 16 integrated circuits having flying leads The 16integrated circuits are attached to each of the circuit boards of thetype found in FIG. 7 through the pressing process previously describedfor circuit board assembly Caul plates of a size slightly larger thanthe circuit boards of the type shown in FIG. 7 are used during thepressing process to attach the integrated circuit chips to the circuitboards. Each plated hole pattern on circuit board 110 of FIG. 7corresponds to the hole pattern disclosed in FIG. 3. Each corner ofcircuit board 110 includes four plated via holes which are used todistribute power and are used for alignment during the pressingoperation.

In the preferred embodiment of the present invention, 16 of the circuitboards 110 shown in FIG. 7 are arranged in a module assembly 200 of thetype shown in FIG. 8. The circuit boards 110 are arranged in a 4 × 4matrix on each level of the module. There are four levels of the modulein which circuit boards are stacked, thus creating an X-Y-Z matrix of 4× 4 × 4 circuit boards. This results in 64 circuit boards for eachmodule assembly 200 which in turn results in 1,024 integrated circuitchips per module assembly.

A module assembly is 4.76 inches wide, 4.22 inches long, and 0.244 inchthick. A top view of a module assembly is shown in FIG. 8. At one edgeof the module assembly are four power blades 201a-201d. These machinedmetal blades are both the mechanical connection to the cabinet intowhich the module assemblies are placed and the electrical connection tothe power supplies. At an opposite edge of the module assembly are 8signal edge connectors 202a-202h. These connectors form thecommunication paths to the other module assemblies within the machine.

Electrical communication between the integrated circuit chips of eachboard 110 is accomplished by means of the prefabricated foil patterns onthe surface and buried within each circuit board. The electricalcommunication between circuit boards 110 is between two logic platessandwiched in the center of the module assembly. Communication betweenthe circuit boards and the logic plates is through gold post jumpersalong the Z-axis direction perpendicular to the planar surface of thecircuit boards and the module assembly. The z-axis jumper wires are usedfor distribution of electrical communication signals and powerdistribution. The Z-axis jumpers are placed in any of the area oncircuit boards 110 that is not occupied by an integrated circuit.

Due to the amount of force required to compress the jumpers along theZ-axis of the module assembly, the jumpers are compressed for a 4-boardstack at one of the 16 locations on the module 200 at a single time. Theorder in which the circuit boards are compressed is shown in FIG. 8 inthe lower left corner of each circuit board stack 110. Sixteen separatepressings are performed to compress the gold Z-axis jumpers for onemodule 200.

FIG. 9 shows a side sectional view of a module assembly. The assembly200 is constructed as a sandwich comprising four layers of circuitboards, two layers of circuit board interconnect layers, and severallayers of support framing material. FIG. 9 depicts a completelyassembled module assembly with the exception of the single edgeconnectors, which have been omitted for purposes of this discussion. Theassembly 200, in application, is stacked with other assemblies in afluid cooling tank and positioned so that the planar surface of themodule assembly is stacked in a vertical direction. Thus, inapplication, the view of the circuit board assembly 200 of FIG. 9 isactually a top-down look at the module in application. A type of coolingapparatus suitable for cooling the circuit board module assemblies ofthe present invention is described in U.S. Pat. No. 4,590,538 assignedto the assignee of the present invention and incorporated herein byreference.

Eight cooling channels 230 are provided at the outer sections of themodule assembly to allow the vertical rise of cooling fluid through themodule assembly to remove the excess heat produced by the integratedcircuits in operation. Heat transfer occurs between circuit boards 1 and4 (levels 212 and 221 respectively) and the fluid passing throughchannels 230. There is also heat transfer from the ends of logic jumpers231 to the passing fluid in channels 230. The latter is the primary heattransfer vehicle from circuit boards 2 and 3 (levels 215 and 218,respectively). The power plates at levels 210 and 223 are spaced fromthe board stacks to form the fluid channels. Spacing is accomplishedwith acrylic strips 203 which are held in place by the power jumpers232.

The module assembly 200 as shown in FIG. 9 depicts one of the four powerblades 201 shown to the left. The outer plates shown as layers 210 and223 are power distribution plates which connect to the four power bladesand are used to distribute electrical power throughout the module forpowering the integrated circuits. The connection between the integratedcircuits and the power plates is by Z-axis power jumpers which aredescribed in more detail below.

As was previously described, each module . assembly consists of 16 boardstacks. Each board stack consists of four circuit boards. The side edgeview of the module assembly shown in FIG. 9 shows four board stacksexposed in a cut-away view. The four circuit board levels are labeledNos. 212, 214, 219 and 221. Electrical communication between theseboards is via two logic plates labeled 216 and 217. These plates are inthe center of the module assembly and divide the board stacks in half.Communication between circuit boards 212, 214, 219 and 221 and logicplates 216 and 217 is via gold post jumpers or logic jumpers 231 in theZ-axis direction (relative to the X-Y axes lying on the planar surfaceof the circuit boards and logic plates). The logic plates as well as thecircuit boards contain electrical interconnecting plated wiring patternsin the X-Y direction, and the Z-direction interconnect is thus performedby the logic jumpers.

The integrated circuit chips 104 are shown in FIG. as the rectangles atlevels 213, 215, 218 and 220. The flying leads from these integratedcircuits are attached to circuit boards 212, 214, 219 an 221respectively. Thus, the circuit board assembly of 212 with integratedcircuits at level 213 are assumed to have been previously assembled withthe aforementioned flying lead attachment of integrated circuits tocircuit boards. The spaces between the integrated circuits at levels213, 215, 218 and 220 contain through-plated holes which are axiallyaligned in the Z-axis direction and allow the gold post logic jumpers231 to pass through the various levels of the module. The spaces betweenthe integrated circuits on levels 213, 215, 218 and 220 are filled witha die frame which also contains corresponding axially aligned holes.This is a clear acrylic plate or block the size of the circuit boardsapproximately 10 mils thick. There are relief areas in the die frame forthe integrated circuit packages and for the gold post jumpers which passthrough the board stacks and through the die frames. The purpose of thedie frame is to provide mechanical support for the circuit boards andfor the gold post jumpers.

The jumpers are forced through the board stack under high pressure tointerconnect all of the axially aligned through-plated holes on thecircuit boards and on the logic plates. The gold jumpers 231 are made ofthe same soft gold used in the flying lead connection of the integratedcircuit packages to the circuit boards described above. The soft goldjumpers are compressed through the axially aligned plated holes to formelectrical connections in the Z-axis direction. The die frame preventsthe soft gold of the jumper from escaping into the areas between thecircuit boards adjacent the integrated circuits.

Jumpers 231 in FIG. 9 are similar to the power jumpers 232 also shown inFIG. 9. The power jumpers 232 extend farther than the logic jumpers 231,since they need to connect to power plates 210 and 223 to supply powerto the circuit boards.

FIG. 10 shows a closeup view of a single logic jumper through thevarious levels of assembly 200. This cross-sectional view of FIG. 10 isnot drawn to scale and is offered as an illustration of how the goldleads are compressed within the plated holes of both the circuit boardsand the logic plates. Spacers are used at levels 213, 215, 218 and 220to prevent the gold jumper leads from expanding into the spaces betweenthe circuit boards. Buried plated interconnect or surface interconnecton circuit boards and logic plates form the interconnection between thelogic jumpers and the plated holes for the flying leads of integratedcircuits. Logic or electrical communication between integrated circuitsand the outside world is achieved therefrom. It will be appreciated bythose skilled in the art that power jumpers will appear similar to thelogic jumpers shown in FIG. 10, except that the power jumpers extendinto the power plates of the assembly 200 and are somewhat larger indiameter.

Gold Post Jumper Installation

The module is assembled in two steps. The first step combines thecircuit board stacks with the logic plates. This step is repeated 16times for a module (once for each board stack). The second step connectsthe power plates and the module power blades. Both steps are describedbelow.

The board stacks are assembled to the logic plates in two pressingoperations. These pressing operations are shown in FIGS. 11 and 12. Thepressing operation shown in FIGS. 13 and 14 presses the power jumpersthrough the assembly to form the necessary interconnect between thecircuit board and logic plate layers and the power plate levels.

The four circuit boards, the four die frames, and the two logic platesare stacked on a metal caul plate with guide pins through the cornerpower jumper holes as shown from the side view of FIG. 11. This is inpreparation for the first pressing operation. There is an assembly dieframe spacer on the lower caul plate before the first circuit board.There is another assembly die frame spacer on the top of the stack justbelow the stamp caul plate, which is removed before the first pressing.

Soft gold post jumpers are then loaded into the stack in the positionswhere the jumpers are desired. These gold posts are in the preferredembodiment 5 mils in diameter and 192 mils long. The top assembly dieframe spacer is removed and is replaced with the stamp caul plate. Theassembly is then placed in a press and the jumpers are compressed suchthat the exposed 10 mils of the logic jumpers are compressed into thestack assembly. In this pressing operation, the 10 mils of the exposedgold post at the top of the stack assembly are compressed into theassembly and the gold posts expand in the jumper cavity to a nearly 100percent fill. Excess gold is forced into a nail head configuration onthe top of the outside circuit board. The assembly in FIG. 11 shows thestamp caul plate on the outer surface of circuit board 4 with the topassembly die frame spacer removed.

The pressing operations shown in FIGS. 11 and 12 are accomplished usinglevelers on the outer surfaces of the logic plates to ensure an evenpressing operation. The guide pins are placed at various points throughthe power jumper holes along the logic plates to ensure that the circuitboards and logic plates do not move during the pressing operation.

FIG. 12 shows the second pressing operation. The board stack shown inFIG. 11 is turned over with the caul plates reversed. In the secondpressing operation, the top assembly die frame spacer on the top side ofthe stack is now removed and another press cycle occurs, pressing theremaining 10 mils of the exposed jumper into the stack. The reason forthe two-sided pressing operation is that the soft gold bindssufficiently in the jumper cavity so that it is not possible to make areliable connection through the entire stack from one side only. Thenumber of pressing operations of course would vary with the number oflevels in the sandwiched assembly. In smaller (thinner) assemblies,one-sided pressing is possible.

With board stacks having a larger number of levels of circuit boards,logic plates and chips, longer logic jumpers and power jumpers may beused to interconnect along the Z-axis, however, more pressing operationsmay be required. For example, in an alternate embodiment of the presentinvention, four pressing operations may be required for logic jumpers.The first pressing operation would be similar to that shown in FIG. 11,except that two 10-mil spacers would be placed at the bottom of theboard stack and two 10-mils spacers placed at the top of the boardstack. The first pressing operation would press 10 mils of the logicjumpers into the board stack after the removal of the top spacer. Thesecond step would start with the removal of the second spacer on thetop, and a second pressing operation would occur. The third pressingoperation would begin with the board stack flipped over and the top10-mil spacer removed for the third pressing step. The last pressingstep would begin with the removal of the final 10-mil spacer and a finalpressing operation would begin. In this application, 20 mils of exposedgold jumper could be pressed into a thicker board stack.

Power plates and module power blades are added to the partial moduleassembly in a manner similar to the pressing operation for theindividual circuit board stacks. In this case, instead of a stamp caulplate of approximately the size of a single circuit board, caul platesthe size of the entire assembly are used to press all of the powerjumpers on the entire module assembly, as shown in FIG. 13. The powerjumpers are loaded and pressed in a four-step cycle as described abovefor the logic jumpers of a single circuit board stack assembly. The goldposts for this power jumper pressing operation are 14 mils in diameterand 284 mils long. The module power blades are attached as a last stepby pressing gold posts or aluminum posts into cavities in the machinedpower blades, as shown in the final step of FIG. 14.

Those of ordinary skill in the art will recognize that other types oflead bonding processes may be substituted for the ball bonding processfor flying leads described herein. Also, other types of malleableelectrically conductive metals may be used in place of the soft golddescribed herein. In addition, the pressing process causing the gold toexpand or buckle within the plated holes may be performed without havingthe gold leads protrude from the circuit board. Compression fingerscould be axially aligned with the plated holes in order to compress thegold leads within the plated holes without having the leads protrudingbefore the process is begun.

While the present invention has described connection with the preferredembodiment thereof, it will be understood that many modifications willbe readily apparent to those of ordinary skill in the art, and thisapplication is intended to cover any adaptations or variations thereof.Therefore, it is manifestly intended that this invention be limited onlyby the claims and the equivalents thereof.

What is claimed is:
 1. A method of fabricating from a length of metalwire a plurality of substantially equal length flying leads which areeach attached to one of a corresponding plurality of bonding pads of anintegrated circuit chip and which extend generally parallel to oneanother and at a predetermined angle from a surface of the integratedcircuit chip, each lead being fabricated by steps comprising:forming aball on an end of the metal wire by rapidly heating the end of the wire;forming an electrical and mechanical connection of the ball to thebonding pad by pressing the ball against the bonding pad of the chip;clamping the wire at a clamping location on the wire located above thesurface of the chip; holding the chip; extending the wire substantiallystraight and at the predetermined angle form the surface of the chipbetween the connection and the clamping location; notching the connectedwire at a notched location at a predetermined height above the surfaceof the chip and between the clamping location and the connection;applying force from the clamping location through the wire and theconnection to the chip; and stretching the wire at a notched location byapplication of the force to server the wire at the notched location andto form the flying lead from the segment of the wire connected to thechip.
 2. A method as defined in claim 1 wherein the extending stepoccurs before the notching step.
 3. A method as defined in claim 1wherein the extending step occurs before the stretching step.
 4. Amethod as defined in claim 1 wherein the notching step does not achievesevering of the wire.
 5. A method as defined in claim 1 wherein themetal wire is substantially gold.
 6. A method as defined in claim 1wherein the predetermined angle is substantially perpendicular to thesurface of the chip.
 7. A method as defined in claim 1 furthercomprising:testing the connection of the ball to the bonding pad priorto the notching step by applying the force to the wire.
 8. A method asdefined in claim 1 wherein the step of forming the connection furthercomprises:heating the chip; heating the ball to a temperatureapproximating the temperature of the chip; and applying sonic energy tothe point of connection of the ball and the bonding pad to scrub theball against the bonding pad while the ball is pressed against thebonding pad.
 9. A method as defined in claim 1 furthercomprising:forming a pointed tip on the free end of the flying lead. 10.A method as defined in claim 9 wherein the step of forming the pointedtip is achieved by notching the wire and stretching the wire to severthe wire at the notched location.
 11. A method of attaching a flyinglead having a predetermined length to an integrated circuit chip,wherein the flying lead is manufactured from an elongated conductivewire longer than the length of the flying lead and having an end, andwherein the integrated circuit chip has a planar surface with at leastone bonding pad on the planar surface, comprising:holding the integratedcircuit chip; forming a ball on the end of the wire; attaching the allto the bonding pad to form an electrical and mechanical bond; clampingthe wire at a clamping location along the wire that is farther from thebonding pad than the predetermined length of the flying lead; stretchingthe wire between the clamping location and the bond perpendicularly tothe planar surface of the integrated circuit chip after the bonding andclamping steps; applying force from the clamping location through thewire and through the bond to the chip to stretch the wire; and severingthe wire at a severing location along the wire between the clampinglocation and the bond as the wire is being stretched, said severinglocation corresponding to the predetermined length of the flying lead.12. A method as defined in claim 11 wherein the step of forming the ballfurther comprises:heating the end of the wire.
 13. A method as definedin claim 11 wherein the wire is substantially gold.
 14. A method asdefined in claim 11 further comprising:extending the wire substantiallystraight and at a perpendicular angle from the surface of the chipbetween the clamping location and the bond prior to stretching the wire.15. A method as defined in claim 11 wherein the step of attaching theball to the bonding pad further comprises:heating the integrated circuitchip; heating the ball; pressing the ball against the bonding pad of theintegrated circuit chip to bring the ball into contact with the bondingpad; and applying sonic energy to the ball to scrub the surface of thebonding pad.
 16. A method as defined in claim 15 wherein the integratedcircuit chip and the ball are heated to approximately the sametemperature.
 17. A method as defined in claim 11 wherein the step ofstretching the wire further comprises:applying the force to the wire atthe clamping location; and resisting the force at the bond between theball and the bonding pad.
 18. A method as defined in claim 17 whereinthe step of severing the wire as it is being stretched furthercomprises:notching the wire at the severing location; and breaking thewire at the severing location by application of the force.
 19. A methodas defined in claim 18 wherein the notching step occurs after the stepof applying the force.
 20. A method as defined in claim 11 furthercomprising:forming a pointed tip on the free end of the flying lead. 21.A method as defined in claim 20 wherein the step of forming the pointedtip is achieved by severing the wire at the severing location.
 22. Amethod as defined in claim 21 wherein the step of severing the wire atthe severing location further comprises:notching the wire at thesevering location; and breaking the wire at the severing location byapplication of the force.